1. Field of the Invention
The present invention relates to a control device for an air hoist which lifts or lowers a load in such a manner that an air motor control valve is operated by a cylinder actuated by a manual throttle valve operable by a worker by hand so that an air motor may be driven to rotate a chain wheel and move a chain.
2. Description of the Prior Art
Air hoists driven by an air motor to rotate a chain wheel include the following types: first, a pull rope manually operated to swing a lever and control an operating valve for the air motor, thereby adjusting the rotational direction and rotational speed thereof to control the lifting speed and the lowering speed; second, a manual throttle valve operated by a worker by hand to actuate an air cylinder, by which a control valve for an air motor is operated; and third, both the pull rope and the manual throttle valve provided with an air hoist having a mechanical brake.
The first type of air hoist is advantageous in that a fine adjustment is possible by operating the operating valve little by little but the operation requires a great amount of skill and also is very dangerous for the worker since he is obliged to work just below the hanging load. Moreover, the mechanical braking action should be successively exerted, thereby avoiding a frictional heat generated to shorten the span of life of a brake.
The second type of air hoist, in which the air cylinder is operated by the manual throttle valve, is operable apart from the load so as to be safer than the first type of air hoist but it is difficult to minutely adjust the speed of the air motor, especially when the load is being lowered.
That is to say, when the air motor is rotated to gradually lower the load while the mechanical brake is released, it is difficult for the worker, who controls the manual throttle valve by hand, to carry out the fine adjustments since the load is hard to lower at slow speed and the air motor tends to rotate quickly, resulting in the danger of lowering the load too fast and the difficulty of stopping the hanging load at the chosen position.
Accordingly, it is an object of the present invention to provide a control device for an air hoist capable of lowering the load in safety and with accuracy by means of an air motor without using a mechanical brake in the lowering phase at either fast or slow speed. The control device for the air hoist is also free from overheating or breaking down of the mechanical brake.
It is another object of the present invention to provide a control device for an air hoist which can perform the lowering of the load at slow or fast speed merely by changing over the lever for the manual throttle valve, thereby efficiently operating the air hoist.
It is still another object of the present invention to provide a control device for an air hoist which can keep the chain stationary without rotation of the air motor even when the worker operates the manual throttle valve for speed reduction control by mistake. Thereby, improvement in the safety of the operation of the invention can be expected.
These and other features of the present invention will become more apparent from the following detailed description with reference to the accompanying drawings.